1. Field of the Invention
The present invention relates to a drill, in which a spiral-shaped chip discharge groove is formed in the outer periphery of the tip section of the drill body, and a production method thereof. In addition, the present invention relates to a drill used for hole drilling in order to form a machined hole at a high level of hole positioning accuracy in a drilled material, and for example, relates to a drill for forming a deep machined hole in a metal material.
2. Background Art
As shown in FIG. 22 through FIG. 24, a known example of a drill (solid drill) in which a spiral-shaped chip discharge groove is formed in the outer periphery of the end section of the drill body is a so-called two-flute twist drill in which a pair of chip discharge grooves 2, which twist to the rear in the direction of rotation of the drill around an axis O towards the rear end from the tip flank of the drill body, are formed symmetrically in the outer periphery of the tip section of a roughly cylindrical drill body 1 that rotates around axis O, and a cutting edge 3 is formed on the intersecting ridge line section between the wall surfaces of these chip discharge grooves 2 facing in the direction of drill rotation T and the aforementioned tip flank. In this type of twist drill, as is indicated in, for example, Japanese Unexamined Utility Model Application, First Publication No. 5-60715, by gradually increasing the groove width of chip discharge grooves 2 from W1 to W2 (>W1) from point A to point B when the outer diameter of the drill is taken to be D and the distance to point A roughly 2D away from the tip of the bit section is taken to be W1, and making it W2 at the section extending from point B to the rear end of the bit section, clogging of chips in the rear end side of chip discharge grooves 2 is prevented, thereby serving to improve the discharge of chips.
In addition, a double margin type drill like that indicated in Japanese Unexamined Patent Application, First Publication No. 7-40117 is known as an example of this type of drill. In this drill, as shown in FIGS. 25 and 26, cutting edge 3 is formed on the intersecting ridgeline section between wall surfaces 2A facing the side of chip discharge grooves 2 in the direction of drill rotation T and tip flank 1A of drilling end 1′ that rotates around axis O, adjacent first and second margin sections 4 and 5, which are adjacent to the rear and front sides in the direction of drill rotation T of chip discharge grooves 2, are respectively formed in land section 1B of drilling end 1′, and these first and second margin sections 4 and 5 fulfill the role guiding drilling end 1′ by contacting the inner wall surface of the machined hole that is formed.
In addition, thinning sections 6 are formed on the tip sides of the inner wall surfaces of chip discharge grooves 2 that are continuous with the inner peripheral edge of cutting edge 3 and is comprised by cutting out a region that extends to land section 1B, including heel section 1C of drilling end 1′, and chips generated by cutting edge 3 are curled by this thinning section 6, thereby serving to improve the discharge of chips.
However, in the drill shown in FIGS. 22 through 24, in the case of gradually increasing the groove width of chip discharge grooves 2 from point A towards point B, as shown in FIG. 22, groove width is increased by extending wall surface 2A facing towards the side of chip discharge grooves 2 in the direction of drill rotation T from the tip side at a fixed helix angle, while widening the wall surface on the opposite side of wall surface 2A, namely wall surface 2B (wall surface on the heel side) that faces towards the rear side of cutting release grooves 2 in the direction of drill rotation T, to the side in the direction of drill rotation T. However, since chips discharged through chip discharge grooves 2 twisted into a spiral shape in this manner are sent to the rear end side while sliding over wall surface 2A so as to be pushed against wall surface 2A facing in the direction of drill rotation T, there is the possibility of the discharge of chips being inadequate simply by widening the side of wall surface 2B on the opposite side of wall surface 2A.
In addition, normally in the production of a drill having chip discharge grooves 2 twisted into a spiral shape in the outer periphery of the tip section of drill body 1, the aforementioned outer periphery is cut into the outer periphery of the tip section of drill body 1 by applying a fixed deflection angle to a grindstone so that the plane that intersects with the center line of the grindstone follows along the direction of twisting of the chip discharge grooves 2 in the case of viewing drill body 1 from the outside in the radial direction, while rotating the disc-shaped grindstone in which an abrasive particle layer is formed on the outer peripheral section around the aforementioned center line, and drill body 1 is then moved along axis O according to the aforementioned helix angle while rotating around axis O (normally the center line of the grindstone is fixed, and drill body 1 is moved along axis O while rotating). As a result, the wall surfaces 2A and 2B are ground to a predetermined shape by the abrasive particle layer resulting in the formation of chip discharge groove 2. In this type of production method, in order to widen the groove width of chip discharge groove 2 to the side of drill direction of rotation T at the rear end side, first grinding is performed over the entire length of chip discharge groove 2 by setting the grindstone to a predetermined deflection angle, and wall surface 2A is formed facing towards the direction of drill rotation T. Next, after shifting the grindstone from the location of the aforementioned point A towards the direction of drill rotation T while maintaining the same deflection angle, drill body 1 is again moved to the rear end side along axis O according to the aforementioned helix angle while rotating around axis O to form wall surface 2B.
However, in this production method, at least two steps are required for respectively grinding wall surfaces 2A and 2B in order to form chip discharge groove 2. In addition, in the case of using the same grindstone for both of these steps, there is the possibility of ridge sections R having a peak-shaped cross-section being formed between the side of wall surface 2A ground in the first step and the side of wall surface 2B ground in the subsequent step in the rear end side from the aforementioned point A as indicated with the broken line in FIG. 24. If this type of ridge section R remains, since the quality of discharge decreases due to chips becoming caught on this ridge section, an additional step is required to remove this ridge section R, thereby resulting in a considerable decrease in drill production efficiency.
In addition, in the case of a conventional double margin type drill, when viewed from the tip side in the direction of axis O as shown in FIG. 25, second margin section 5, which is formed so as to be adjacent to the front side in the direction of drill rotation T of chip discharge groove 2 in land section 1B of drilling end 1′, is only present in the extremely small region facing towards the front side in the direction of drill rotation T from heel section 1C of drilling end 1′. Consequently, in the case of having formed a large thinning section 6 that reaches to land section 1B that includes heel section 1C, as shown in FIG. 26, the tip of second margin section 5 recedes considerably towards the rear end side by the amount thinning section 6 is formed, thereby making the distance along axis O between the tip of first margin section 4 and the tip of second margin section 5 extremely large.
However, as shown in FIGS. 27A and 27B, machining in which a machined hole K to be formed is opened towards a position shifted from center C1 of a cross hole C in the cross hole C formed in advance in a drilled material is an example of drilling using a double margin type of drill. In this case, when drilling end 1′ passes through to the inner wall surface of cross hole C, force is applied in the horizontal direction (direction X in the drawing) that intersects with axis O to the tip section of drilling end 1′. However, in the case of a conventional double margin type drill as was previously described, since second margin section 5 is made to recede considerably towards the rear end side due to the presence of thinning section 6, second margin section 5 is unable to make contact with the inner wall surface of the exit section of the formed machined hole K for a short time after drilling end 1′ passes through the inner wall surface of cross hole C (the time until the feed amount to the tip side in the direction of axis O imparted to drilling end 1 becomes the aforementioned distance h). As a result, even though first and second margin sections 4 and 5 were formed to guide drilling end 1′ in a stable manner, during this time, drilling end 1′ is only guided by first margin section 4, thereby causing to be guided in an unstable manner.
Consequently, drilling end 1′ is unable to be guided in a stable manner with respect to force from direction X in FIGS. 27A and 27B, resulting in the occurrence of runout in drilling end 1′ and causing problems such as increased surface roughness of the inner wall surface of the formed machined hole K or chipping cutting edge 3 due to contact with the wall surface of machined hole K (and breaking drilling end 1′ in cases of particularly excessive runout).
In addition, in the case of conventional drills, wear resistance may be improved by covering the surface of drilling end 1′ (including the surfaces of tip flank 1A of drilling end 1′, land section 1B and the inner wall surfaces of chip discharge grooves 2) with a hard coating such as TiN or TiCN.
However, since the surface roughness of these hard coatings is comparatively large at 2-4 μm, if first and second margin sections 4 and 5 that contact the inner wall surface of machined hole K are covered with a hard coating having such a large surface roughness, the surface roughness of the inner wall surface of machined hole K increases accompanying contact with first and second margin sections 4 and 5. In particular, the increase in surface roughness on the inner wall surface of machined hole K becomes prominent in the initial stage of drilling during which there is the absence of the phenomenon in which the surface roughness of first and second margin sections 4 and 5 becomes smaller due to fraction with the inner wall surface of machined hole K. In addition, although chips generated by drilling are fed out and discharged to the rear end side while sliding over the inner peripheral surface of the chip discharge groove 2 from the rake face that is located on the tip side of the section that faces towards the front in the direction of drill rotation T, if the inner peripheral surface of chip discharge groove 2 is covered with a hard coating having a comparatively large surface roughness as mentioned above, resistance increases and the quality of chip discharge decreases. As a result, chips are easily clogged in chip discharge groove 2, and when this clogging becomes prominent, it may lead to breakage of drilling end 1′.
Moreover, the drilling of deep holes, which had been conventionally performed with a gun drill, has recently come to be frequently performed using a type of drill in which the total length of bit section 1′ extends to 10×D to 20×D, and even 25×D depending on the particular case, with respect to external diameter D of the drill in order to improve drilling efficiency. However, in the case of a drill such as this for drilling deep holes in which the total length of bit section 1′ is long, since the distance over which chips generated by cutting edge 3 are discharged through chip discharge groove 2 also becomes longer, chips easily become clogged in chip discharge groove 2. Moreover, since drill rigidity and strength also tend to decrease as the total length of bit section 1′ increases, the drill is increasingly susceptible to breakage due to clogging of chips in chip discharge groove 2.
Moreover, in the case of conventional drills, back tapers are attached to first and second margin sections 4 and 5 that contact the inner wall surface of machined hole K so that the outer diameter of drilling end 1′ gradually decreases at a constant ratio moving towards the rear end side in order to reduce the contact surface area with the inner wall surface and decrease drilling resistance.
However, since back tapers are attached over the entire length of drilling end 1′, when large back tapers are attached in order to impart adequate clearance to the outer peripheral surface of drilling end 1′, the outer diameter of drilling end 1′ becomes smaller than necessary at its rear end section, thereby causing the problem of decreased rigidity of drilling end 1′. This is particularly conspicuous in the case of a drill used for forming deep machined holes, namely a drill in which the total length of drilling end 1′ is long.
On the basis of this background, an object of the present invention is to provide a drill that reliably and further improves the discharge of chips while effectively preventing the clogging of chips, while also providing a production method of a drill that enables such a drill to be produced without decreasing production efficiency.
In addition, an object of the present invention is to provide a drill that allows the obtaining of stable guiding action for the drilling end by first and second margin sections even in the case of a double margin type drill in which a large thinning section is formed.
In addition, an object of the present invention is to provide a drill capable of preventing the clogging of chips and drill breakage by decreasing resistance during chip discharge, while also being capable of reducing the surface roughness of the inner wall surface of a machined hole formed by drilling, even in the case of having improved wear resistance by covering the surface of the drilling end with a hard coating.